In the area of technology of electroerosive machining, particularly for very small structures (micro-spark erosion), there is the acute problem that high aspect ratios are needed. This means that the required ratio of the structural depth to the (smallest) lateral strain is often very high (10 or more). As the depth of the erosion increases there is the problem that, as a result of decreasing dielectric circulation in the base of the bore, eroded particles tend to accumulate which then impede a defined spark arc-over. This causes a slowing of the boring process and increased wear on the electrode, as well as degradation of the structural precision of the bore. The literature discloses a number of approaches to mitigate these effects.
E.g., DE 3032604 proposes the use of hollow tubular electrodes for production of bores, wherewith the dielectric is fed to the base of the bore under pressure through the interior of the electrode. This method is of limited effect with bores of very small diameter. For an electrode internal diameter of less than 70 microns, the pressure drop over the length if the electrode is so high that practically no more dielectric can be forced through it. Pressurized rinsing is thus not an option in this situation.
In DE 3032604 it is proposed to support pressurized rinsing by means of vibrational excitation of the electrode at high or low frequency. Vibration of the electrode can assist the process because the dielectric is caused to move in the working gap and thus holds the erosion particles in suspension. In order to produce very small structures (as small as less than 10 micron) with maximum possible accuracy, very high requirements are placed on the drive device. This technique has promise but awaits further means of improvement.
It is known from FR 2577156 to transmit a vibrational movement in the advancing direction of the electrode with a rotational movement of the electrode along its longitudinal axis. The reported effects of this represent another improvement. However, the drive device disclosed in that patent document is insufficiently precise, particularly as to its vibrational drive and guide unit, to succeed in producing structures of very small dimensions. The connection to an external rotational drive is furnished by a cone on the upper side of the device. The axially acting linear drive operates via a plunger coil, wherewith the drive shaft has a coupling in the form of spring elements, in order to transmit the rotational movement and to provide a degree of freedom for the axial oscillation. The rigid part of the drive shaft is supported by grooved ball bearings, and the axially movable part of the drive shaft is radially supported against a housing by means of a sliding bearing.
Because the present device is intended to be suitable for micromachining in particular, high requirements are placed on the accuracy of its concentric alignment. Rotational tolerances in the submicron range are not achievable with the device according to FR 2577156 because of imprecision of the guiding- and drive components.
JP 3060928 and JP 60255323 disclose drive devices for erosion tools, having a plurality of piezo actors by means of which an electrode holder with its electrode can be displaced in all three spatial dimensions, and in particular in the case of JP 60255323 can be tipped so as to deviate the orientation of the longitudinal axis. The tipping serves to correct possible deviation of the electrode with respect to the direction of advance. JP 3060928 proposes to employ the piezo drive to support the process, in particular to move the electrode back and forth in the direction of advance. Nothing is said concerning the precision of the structure, particularly the precision of the guide unit, for purposes of achieving high precision in the machining.
EP 0636443 has the objective of improving erosion conditions by causing high frequency microvibrations to act on the electrode holder, the amplitudes of the vibrations being smaller than the spark gap at the base of the bore. Preferably, piezo actors are utilized for this effect; they act upon an elastic metal block. The system is designed for operation in a resonant state. For this reason, and by reason of the principle of the guide means disclosed in the document, there is no assurance that the electrode will move parallelly to the longitudinal axis.
DE 2811274 A1 discloses a device for controlling the electrode of a spark erosion machine having adjustable eccentricity of the electrode holder. The electrode holder is connected to a shaft which on its outer periphery is mounted in the housing of the device, via a bushing and a ball bushing.
DE 1237713 B discloses a drive device for an erosion electrode wherein a drive rod is moved back and forth in the axial direction by a leading spindle, and is simultaneously rotated around the longitudinal axis by engagement in a spiral path in a cam tube (with guide means for a cam). The drive rod is mounted in a frame by means of two ball bushings which enable the superposed axial and rotational movement.
CH 350735 A relates to a holding and drive device for erosion tools which is rotatable around an axis and is slideably displaceable in the axial direction. The guiding in the axial direction is by means of two parallel guide tubes each of which has an interior tube, wherewith a ball bearing with race is disposed between the respective guide tube and inner tube, for practically play-free and frictionless mounting of the interior tube. The two interior tubes are rigidly interconnected via a connecting arm. One interior tube has a drive shaft for the electrode holding device, which shaft is movable therein via a needle bearing and two ball bearings which absorb axial as well as radial forces.
EP 0264147 also discloses devices for fine adjustment by means of piezo actors. These are incorporated in elastic monolithic blocks which allow movements in multiple dimensions. The use and conversion of such adjusting drives as drive devices for erosion tools is not discussed in any appreciable detail. Such systems do allow exact axial guiding [and positioning]. However, they are costly to produce and impracticable to produce on any useful scale.
Another drive and bearing concept is disclosed in EP 1473103 A1. All of the bearings and positioning drives employ electromagnetic means. According to an exemplary embodiment, two electromagnetic positioning drives disposed an axial distance apart are provided for radial (x-y) spindle displacement. Further, there are two axially acting positioning drives (z-displacement) for the spindle, and these can be used also to produce axial vibration. Also, an electromagnetic rotational drive is provided which acts directly on the spindle. Finally, there are two mutually axially separated electromagnetic auxiliary bearings which limit the excursion of the radial spindle displacement. The described drive device is the first that combines the initially described features.
The known drive concepts are beset with certain problems, as a result of the contactless electromagnetic actors; this is particularly true of drive devices with externally coupled rotational drive means through which, as a rule, undesirable transverse forces are conducted into the drive device. The disclosed bearing principle for the spindle allows radial vibration modes to form in the spindle, and these are disadvantageous as to the necessary precision.